Approximate joint measurement of qubit observables through an Arthur–Kelly model

We consider a joint measurement of two and three unsharp qubit observables through an Arthur–Kelly-type joint measurement model for qubits. We investigate the effect of the initial state of the detectors on the unsharpness of the measurement as well as the post-measurement state of the system. Particular emphasis is given on a physical understanding of the POVM to PVM transition in the model and entanglement between the system and the detectors. Two approaches for characterizing the unsharpness of the measurement and the resulting measurement uncertainty relations are considered. The corresponding measures of unsharpness are connected for the case where both the measurements are equally unsharp. The connection between the POVM elements and symmetries of the underlying Hamiltonian of the measurement interaction is made explicit and used to perform the joint measurement in arbitrary directions. Finally, in the case of three observables, we derive a necessary condition for the approximate joint measurement and use it to show the relative freedom available when the observables are non-orthogonal. (paper)